1. Field of the Invention
The present invention relates to a process and an apparatus for the deposition on a substrate of a thin layer of material having a stoichiometric composition, particularly suited for use as an electrochromic layer with the highest colouring efficiency.
2. Description of the Prior Art
Devices known as electrochromic cells are known from the prior art, and have the ability to change colour when an electric current passes through them, the intensity of the colour being correlated to the total amount of the electric charges transferred.
If said electrochemical electrochromic systems show no parasite chemical reactions inverting colouration autonomously, the coloured layer of the cell will remain intact until an equal amount of charge, of the opposite sign, causes it to invert.
The colouring efficiency of an electrochromic layer is indicated with E, at a particular light wave length L, and is defined as the variation D in optical density OD, per passed charge density unit, that is to say E(L)=D OD/mC cm-2, in which the charge density unit is in millicoulombs (mC) per square centimeter (cm-2) and in which the optical density OD is the base 10 logarithm of the ratio between initial transmittance and the transmittance obtained.
The electrochromic systems can be used as glazing for automobiles and buildings, as large displays for motorway indicator signs or as small tables for shelving units, as devices for use in credit cards or for electrical indicators.
There are also applications for this type of material that do not make use of its colouring properties, but on the contrary make use of their electronic and thermodynamic properties, a typical example of which are thin film batteries or electronic switches.
The electrochromic systems taken into consideration all use at least one electrode which is coated either wholely or partially with a thin layer of metal oxide, called the host metal, which has the ability to change colour when an atom of suitable material, such as lithium, sodium, silver, potassium, magnesium or hydrogen is inserted into said material.
These host materials are typically tungsten trioxide (WO3), or molybdenum trioxide (MoO3) or a solid solution of oxides of these materials alone or together with other oxides of transition metals.
In addition to these colourable host materials there are the so-called counter-electrodes which are colourless or coloured in a complementary manner to the coloured host material.
Thus, for example, a colourless counter-electrode material is vanadium pentoxide (V2O5). If this counter-electrode contains a monovalent atom such as, for example, lithium at a suitable level of concentration, then the original yellow-green colour is neutralized thanks to the corresponding movement of the absorption band due to the presence of the lithium atom.
The overall chemical composition of this colourless material is LixV2O5.
Another colourless counter-electrode material is a solid solution of V2O5 and MoO3 at a suitable concentration ratio.
Said thin film counter-electrodes must have a sufficient quantity of contained atoms, for example of lithium, hydrogen, sodium, silver, potassium or magnesium so that when a fraction of these atoms is transferred to the electrode to be coloured to obtain the desired electrochromic change there is no visible change in the appearance of the counter-electrode.
If the atoms are removed until reaching the limit at which the layer becomes coloured, then the resulting change of colour will increase the colour change in the main electrode.
The latter situation can be determined by having a sufficiently thin film so that substantially all the atoms are transferred from the counter-electrode to the main electrode during the colouring phase and vice versa during the decolouring phase, as has been described in the literature (Green, Evans, Hussain, "Optical properties of thin film oxide bronzes for window applications, Second International Symposium on Polymer Electrolytes, Ed. B. Scrosati, Pub. Elsevier Applied Science, London, p449-1989; European patent application No. 0608203).
The layers described above, made up of host material or counter-electrode material, are indicated in the present description as electrochromic layers.
Said electrochromic layers are applied to a number of conductor electrodes, at least one of which is transparent, which are typically plastic or glass coated with doped tin oxide, or a mixture of tin and lithium oxide and in the case of use for glazing purposes all the electrodes are transparent.
In other applications, such as, for example, in credit cards, the transparent electrode can be of plastic material coated with conducting tin oxide and then coated with an electrochromic layer.
The composition and structure of these electrochromic layers is critical for the operation of the electrochromic device.
In fact it is known from the state of the art that the electrochromic material forming said electrochromic layers must be without humidity, with an extremely fine polycrystalline grain and with a stoichiometric composition as described in the English patent applications No. 2081922 and No. 2164466.